To understand the workings of the heart in depth it is necessary to study the cardiac muscle mechanics free from structural complexity and inhomogeneity of the myocardium. To this end, use of cardiac myocytes clearly offers many advantages over other preparations such as papillary muscle or whole heart. Many investigators have attempted to use the myocyte as a preparation of choice but technical problems prevented them from obtaining satisfactory results. The myocyte is very small (typically 15 mum wide and 100 mum long) and extremely difficult to use for mechanical experiments because of absence of tendinous ends which serve as mechanical attachments to apparatus. This research is aimed at unraveling complete mechanical properties of the myocyte in passive and active states, particularly in the lengths where thin filament double overlap takes place. The study involves measurements of forces, sarcomere length, dynamic responses and stiffness spectra up to 50 KHz. Measurements of forces and displacements with unprecedented sensitivity and wide bandwidth have recently become possible owing to development of new instrumentation and special techniques. These were originally aimed at measuring the mechanics of single myofibrils, but the methodology was recently modified to suit for myocytes and ultra-thin trabeculae. New experimental techniques tailored for myocytes have also been developed to allow compression of the preparation without buckling thus making it possible to study the mechanic at the lengths below resting length, an area where no satisfactory experiment has been done before despite the fact that short sarcomere lengths are where cardiac muscle performs the most work. The proposed study deals with skinned myocytes and trabeculae so that their chemical environment is directly controlled to establish relationships between mechanical properties and chemical parameters. The study includes force-length, force-velocity, and dynamic stiffness measurements in both passive and active states at sarcomere lengths encompassing entire working range of the heart. There have already been some unexpected results in the preliminary studies which may be significant in understanding the molecular mechanism involved in short sarcomeres. The proposed study will pave a way to the second phase of the study which will include membrane system by using intact myocytes and trabeculae so that the effects of the E-C coupling is integrated into the mechanics.